Helps you to save uploaded files to disk in ASP.Net Web API. The SaveFile method works with Request object directly.

AspnetWebApiUploadFile.cs

public async Task<IHttpActionResult> SaveFile(string diskFolderPath)
        {
            var path = Path.GetTempPath();

            if (!Request.Content.IsMimeMultipartContent("form-data"))
            {
                throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.UnsupportedMediaType));
            }

            MultipartFormDataStreamProvider streamProvider = new MultipartFormDataStreamProvider(path);

            await Request.Content.ReadAsMultipartAsync(streamProvider);

            foreach (MultipartFileData fileData in streamProvider.FileData)
            {
                string fileName = "";
                if (string.IsNullOrEmpty(fileData.Headers.ContentDisposition.FileName))
                {
                    fileName = Guid.NewGuid().ToString();
                }
                fileName = fileData.Headers.ContentDisposition.FileName;
                if (fileName.StartsWith("\"") && fileName.EndsWith("\""))
                {
                    fileName = fileName.Trim('"');
                }
                if (fileName.Contains(@"/") || fileName.Contains(@"\"))
                {
                    fileName = Path.GetFileName(fileName);
                }

                var newFileName = Path.Combine(HostingEnvironment.MapPath("~" + diskFolderPath), fileName);
                var fileInfo = new FileInfo(newFileName);
                if (fileInfo.Exists)
                {
                    fileName = fileInfo.Name.Replace(fileInfo.Extension, "");
                    fileName = fileName + (new Random().Next(0, 10000)) + fileInfo.Extension;

                    newFileName = Path.Combine(HostingEnvironment.MapPath(diskFolderPath), fileName);
                }

                if (!Directory.Exists(fileInfo.Directory.FullName))
                {
                    Directory.CreateDirectory(fileInfo.Directory.FullName);
                }


                File.Move(fileData.LocalFileName, newFileName);

                return Json(new { link = diskFolderPath + fileName });
            }

            return BadRequest();
        }

using System;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using System.Web.Hosting;
using System.Web.Http;

namespace WebApi.Controllers
{
    public class BaseController : ApiController
    {
        public async Task<IHttpActionResult> SaveFile(string diskFolderPath)
        {
            var path = Path.GetTempPath();

            if (!Request.Content.IsMimeMultipartContent("form-data"))
            {
                throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.UnsupportedMediaType));
            }

            MultipartFormDataStreamProvider streamProvider = new MultipartFormDataStreamProvider(path);

            await Request.Content.ReadAsMultipartAsync(streamProvider);

            foreach (MultipartFileData fileData in streamProvider.FileData)
            {
                string fileName = "";

                if (string.IsNullOrEmpty(fileData.Headers.ContentDisposition.FileName))
                {
                    fileName = Guid.NewGuid().ToString();
                }
                else
                {
                    fileName = fileData.Headers.ContentDisposition.FileName;
                    if (fileName.StartsWith("\"") && fileName.EndsWith("\""))
                    {
                        fileName = fileName.Trim('"');
                    }
                    if (fileName.Contains(@"/") || fileName.Contains(@"\"))
                    {
                        fileName = Path.GetFileName(fileName);
                    }
                }
                var newFileName = Path.Combine(HostingEnvironment.MapPath("~" + diskFolderPath), fileName);
                var fileInfo = new FileInfo(newFileName);
                while(fileInfo.Exists)
                {
                    fileName = fileInfo.Name.Replace(fileInfo.Extension, "");
                    fileName = fileName + Guid.NewGuid().ToString() + fileInfo.Extension;

                    newFileName = Path.Combine(HostingEnvironment.MapPath(diskFolderPath), fileName);
                    fileInfo = new FileInfo(newFileName);
                }

                if (!Directory.Exists(fileInfo.Directory.FullName))
                {
                    Directory.CreateDirectory(fileInfo.Directory.FullName);
                }


                File.Move(fileData.LocalFileName, newFileName);

                return Json(new { link = diskFolderPath + fileName });
            }

            return BadRequest();
        }
    }
}

Here I fixed some bugs like creating random filename and else statement